MRI devices are diagnostic imaging devices for medical use which obtain, from a magnetic resonance signal that is generated by making atomic nuclei in an arbitrary cross-section across a test object cause magnetic resonance, a cross-sectional image in the cross-section. Generally, in MRIs, a magnetic resonance signal which is generated from a hydrogen nucleus (1H) is a measurement object. However, the MRIs can also measure nuclear magnetic resonance signals of many nuclides such as fluorine (19F), phosphorus (31P), sodium (23Na), and carbon (13C).
For example, in order to simultaneously obtain images that are formed from nuclear magnetic resonance signals of a plurality of kinds of different atomic nuclei, it is necessary to apply a high-frequency magnetic field to excite the atomic nuclei, and to tune an RF coil which detects a nuclear magnetic resonance signal to frequencies (magnetic resonance frequencies) of the nuclear magnetic resonance signals of the plurality of atomic nuclei. Such a coil refers to a multi-tuned RF coil. Particularly, an RF coil which can receive nuclear magnetic resonance signals of two kinds of atomic nuclei refers to a double-tuned RF coil (for example, see PTL 1).
Common RF coils are required to have a high signal to noise ratio (SNR) in order to deal with weak signals. In response to this, there is a multi-element RF coil in which a plurality of RF coils are disposed and simultaneously driven.
As one multi-element RF coil, there is a phased array coil which has a plurality of RF coils arranged side by side to simultaneously receive signals.
In multi-element RF coils, magnetic coupling due to mutual magnetic induction is mainly generated between RF coils. This magnetic coupling causes image artifact and the like. Accordingly, it is necessary to prevent or suppress the magnetic coupling. For example, the following two methods are used to prevent or suppress the magnetic coupling (hereinafter, referred to as elimination of the magnetic coupling).
A first magnetic coupling elimination method is a method in which some of coil parts which transmit and receive high frequencies of a plurality of RF coils are disposed to overlap each other. In this method, mutual magnetic induction between two adjacent RF coils can be eliminated by appropriately adjusting an overlapping amount.
A second magnetic coupling elimination method is a method using a low-input impedance pre-amplifier (for example, see NPL 1). In this method, a high impedance is provided to both ends of a coil part by setting the low-input impedance pre-amplifier, an inductance which connects RF coils, and a matching capacitor of an RF coil on the condition that these resonate in parallel at a magnetic resonance frequency. As a result, a current having a magnetic resonance frequency flowing in the coil is reduced, and mutual magnetic induction between the coils can be eliminated.
In the case of the first magnetic coupling elimination method, the degree of freedom of the arrangement of the RF coils is low, and thus it is difficult to optimally adjust the arrangement to balance the elimination of the magnetic coupling with the improvement in receiving sensitivity. In addition, in the case of the second magnetic coupling elimination method, its elimination ability is lower than that of the first magnetic coupling elimination method. Particularly, the elimination function is not sufficient when strong magnetic coupling is generated between adjacent RF coils. Accordingly, in general, in the case of a phased array coil, the above methods are used in combination and the magnetic coupling is eliminated.